1. Field of the Invention
This invention relates to an electrophotographic device which forms an electrostatic latent image by way of electric charging and exposure steps. More particularly, it is concerned with an electrophotographic device which is capable of stabilizing the potential of the electrostatic latent image to produce a stable image.
2. Description of Prior Arts
Generally speaking, an electrostatic latent image can be formed by (1) a method of uniformly charging a photosensitive member constructed with a photoconductive layer coated on an electrically conductive substrate by a corona discharge followed by irradiation of a light image (the so-called "Carlson Process"); or (2) a method of charging a photosensitive member constructed with a photoconductive layer formed on an electrically conductive base and an insulative layer further provided on the photoconductive layer with a corona discharger as disclosed in U.S. Pat. No. 4,071,361 or 3,666,363, then removing the charge by a corona discharge in a polarity opposite to that of the abovementioned corona discharge or an a.c. corona discharge substantially simultaneously with the light image irradiation, and finally uniformly irradiating light over the entire surface of the photosensitive member.
In either method, however, the photoconductive substance to be used has a phenomenon called "photo-hysteresis". On account of this phenomenon, when the photosensitive member is repeatedly used, there occurs a difference in the latent image potential between a portion where light was irradiated previously and a portion where no light was irradiated, this potential difference emerging as the so-called "ghost" in the image after its development. In order to solve this problem, there has generally been adopted a method, wherein the photosensitive member is subjected to uniform light irradiation over its entire surface prior to entering into the charging process after the latent image has been developed, thereby eliminating the abovementioned ghosting phenomenon. Further, the photoconductive substance has such a phenomenon that, according as it is composed generally of a higher and higher resistance material, the smoothness in the image quality on the solid black portion becomes lowered (the so-called "coarseness"). The abovementioned light irradiation prior to the abovementioned charging is effective for solving this problem. This uniform light irradiation over the entire surface of the photosensitive member will hereinafter be called "pre-exposure". It has already been experimentally verified that the light quantity for the pre-exposure for solving the abovementioned ghost and coarseness phenomena may be at a predetermined quantity or above. Also, when the latent image formation is continuously done, the photosensitive member, due to its own characteristic, increases the latent image potential with lapse of time (the rising characteristic) or decreases the same with lapse of time (the trailing characteristic), even if the exposure light quantity and the output from the charger are constant. This rising and trailing characteristics give mal-effect to the image quality. For example, when the photosensitive member has the rising characteristic, the density gradually increases and the so-called "fogging" occurs on the white background portion. In the case of the trailing characteristics, the density gradually decreases and the image skips off from place to place.
In order therefore to compensate these rising and trailing characteristics, there has so far been contemplated control of the exposure light quantity, charging quantity, developing bias, and so forth. However, the control of the charging quantity and the developing bias invites complicacy and increase in size of a high tension transformer, which is inconveniently associated with high manufacturing cost and increased size of the electrophotographic device as a whole. Moreover, since variations in quantity of the electric potential at both dark and light portions on the photosensitive member differ depending on the rising or trailing characteristics, such characteristics cannot be corrected perfectly by the control of the charging quantity alone. Also, there is a method of controlling the developing bias voltage as a voltage which is slightly increased to a value of the potential at the light portion, although this cannot be a fundamental solution to variations in the image contrast (a differential voltage between the light portion potential and the dark portion potential). Furthermore, it has been difficult to correct variations in the latent image potential of the photoconductive substance by the control of the image exposure light quantity alone.
Incidentally, sensitivity characteristics of the device, that is the rising and trailing of the potential in the photoconductive substance such as Se, CdS, ZnS, and so on (hereinafter referred to "photosensitive member"), are not uniform. Therefore, the factors causing the variations in the latent image potential are of variety, as follows.
(A) rising and trailing characteristic of the potential at the dark portion (a portion in the photosensitive member where no light is irradiated, i.e., a portion corresponding to black image in an original);
(B) rising and trailing characteristic of the potential at the light portion (a portion in the photosenstive member where light is irradiated, i.e., a portion corresponding to white image in an original);
(C) change in the degree of the preceding sensitivity characteristics (A) and (B) due to stoppage time of the device;
(D) change in the degree of the preceding sensitivity characteristics (A) and (B) due to the potential state prior to stoppage of the device;
(E) change in the degree of the preceding sensitivity characteristics (A) and (B) due to fluctuations in the characteristics at the time of manufacturing the photosensitive member; and others.
It has been difficult to provide the electrophotographic device which is capable of responding to all the afore-described factors for the potential changes in the latent image by the conventional technique.